We wish to define the events necessary for the transition, in response to specific mitogens, from non-cycling Go cells to proliferation cells. We use murine 3T3 cells and well-characterized mitogens to study this transition. Diverse agents of vastly different chemical structure are mitogens for 3T3 cells. A variety of events occur as a consequence of mitogenic stimulation of 3T3 cells, including increased transport of K ion, Na ion, and uridine, increased activities or ornithine decarboxylase and phosphofructokinase, receptor-ligand internalization (in some cases), etc. We are concerned with two major questions: (1) For specific mitogens, which are "causal mitogenic events" necessary for the Go yields Gl transition and which are only " correlative mitogenic events", accompanying the Go yields Gl transition but not required for mitogenic response? (2) What are "ubiquitous mitogenic events" requisite for the action of all mitogens, and what are "unique mitogenic events" occuring only in response to a single mitogen or subclass of mitogens? We developed a technique for the selection of mitogen-specific non-proliferative variants and have now isolated (1) independent 3T3 variants which are not stimulated to divide by epidermal growth factor (EGF) but which do respond to other mitogens, and (2) independent 3T3 variants which are not stimulated to divide by the co-carcinogen tetradecanoyl phorbol acetate (TPA) but do respond to other mitogens. Within each class (EGF non-responders and TPA responders) there exist variants which retain interaction with ligand, i.e., variants which apparently have "post-receptor" defects in the proliferation pathway. We plan (1) to isolate additional EGF and TPA non-responders as well as non-responders for the polypeptide mitogens vasopressin and fibroblast growth factor, (2) to characterize the defects in our mitogen-specific non-proliferative variants, (3) to isolate temperature-sensitive (i.e., conditional) mitogen-specific non-proliferative variants, (4) to isolate variants unable to internalize EGF, in order to evaluate the role of internalization in EGF mitogenesis, (5) to initiate a somatic cell genetic study of EGF receptor biosynthesis, and (6) to initiate a somatic cell genetic analysis of complementation groups required for the action of specific mitogens.